Determining protective layer thickness of blast furnaces

ABSTRACT

A method is provided for determining the thickness of a protective layer of solidified metal skull formed on the refractory hearth of a blast furnace. The refractory hearth has temperature probes embedded in the floor and walls thereof. The method includes periodically measuring temperatures indicated by the probes and determining the campaign maximum and current average temperature readings to locate two solidification isotherms representing the wear line of the refractory and the inner surface of the protective metal layer. The thickness of the protective layer is determined from the distance between the solidification isotherms representing the refractory wear line and the inner surface of the metal skull.

This is a Division of Application No. 08/938,760, filed on Sep. 26,1997, now U.S. Pat. No. 5,890,805.

TECHNICAL FIELD

The present invention is of a method for extending the life of a blastfurnace refractory lining and, and particularly to a method whichincludes on-line monitoring of campaign maximum and current averagesignals from a plurality of thermocouples embedded at spaced locationsin the refractory lining and from a plurality of thermocouples embeddedat spaced locations in a metal shell of the furnace, calculating fromthose signals the wear line of the refractory lining and the thicknessof a layer of solidified metal skull formed on an inner surface of therefractory, and then determining the conditions of heat transfer at theshell e.g. whether or not a gap has formed between the refractory andthe shell and whether or not water cooling of the shell is sufficient.

BACKGROUND ART

The iron blast furnace typically is constructed of a metal shell with arefractory brick lining. The life of the refractory brick liningdetermines the length of time that the furnace can be kept in operationbefore the furnace must be shut down for installation of new refractory.Longer refractory life decreases refractory cost and increases theproductivity achieved from the furnace. More expensive refractory brickhave been used to extend the length of a furnace "campaign". Grouting orgunning of refractory material between the refractory brick and themetal shell has also been used as a repair measure to close the gapswhich sometimes form between the shell and the brick. Gaps between thebrick and shell decrease heat transfer and cause increased wear of therefractory brick.

U.S. Pat. No. 4,510793 discloses the use of a ceramic bar in a furnacewall which wears with the lining. The wear of the bar and the lining isdetected ultrasonically by generating ultrasonic pulses in the bar anddetecting the reflection of the pulses from the worn inner end of thebar.

Japanese Published Application 1-290709 discloses thermocouples embeddedin the refractory on the bottom and bottom side wall part of a blastfurnace. From the temperatures measured by the thermocouples,calculations are made to determine the state of packing of coke in thecore of the furnace. When the packing of coke is inadequate forpreferential flow of molten iron in the central part of the furnace,changes are made in the amount, grain size or hot characteristics of thecoke charged to the furnace.

U.S. Pat. No. 4,358,953, Horiuchi et al, discloses a method ofmonitoring the wear of refractory lining blast furnace walls by sensingtemperatures at different points across the thickness of the refractoryand analyzing the time delay between trigger signals representinginternal phenomena of the furnace and the temperature probe outputsignals. This patent also describes a prior art method of determiningthe degree of wear from one dimensional heat transfer analysis. Anapparatus for sensing temperature distribution in the refractory is alsodisclosed. A similar apparatus is disclosed in U.S. Pat. No. 4,412,090,Kawate et al, and in U.S. Pat. No. 4,442,706, Kawate, et al.

It is also known to use one-dimensional and two-dimensional heattransfer calculations to refractory temperature distributions and thenlater compare with measured temperatures to estimate remainingrefractory and skull thickness. A method of this type is disclosed in aliterature paper entitled "Evaluation of Mathematical Model forEstimating Refractory Wear and Solidified Layer in the Blast FurnaceHearth", by Suh Young-Keun et al, ISIJ, 1994, Pages 223-228. However, nomethod previously existed for using measured temperatures to calculatethe thickness of the brick and skull directly in a manner whichconsiders interaction between measured temperatures at all locations ina vertical plane simultaneously. Also, no previous method existed thatcould be used on-line without human intervention to signal problems withgap formation, inefficient cooling on the shell, and to discern theirregular "elephant-shaped" erosion profiles and "bowl-shaped" erosionprofiles, so as to enable corrective measures to be taken during furnaceoperation in order to extend the life of the refractory.

Other patents related to the measurement of wall thickness and/ortemperature include U.S. Pat. Nos. 2,264,968; 2,987,685;2,994,219;3,018,663; 3,307,401; 3,512,413; 4,217,544; 4,248,809; and4,539,846.

DISCLOSURE OF THE INVENTION

This invention is of a method for extending the life of refractorylining the interior of a metal shell of a blast furnace. The methodincludes placing thermocouples in the refractory at a plurality ofspaced locations and monitoring the signals produced by thethermocouples during furnace operation. An average of the temperaturereadings at each thermocouple location is determined periodically andrecorded. The maximum temperature reading since the beginning of acampaign of the furnace is also determined and recorded. From thecurrent average and campaign maximum temperature readings from thethermocouple signals, a determination is made On-line, i.e. duringfurnace operation, as to whether a protective solidified layer of metalskull exists on the inner face of the refractory and the thickness ofthe skull. If no protective layer of solidified metal skull exists, orif it is of insufficient thickness, a determination is made as towhether a gap exists between the refractory and a metal shell of thefurnace and the location of the gap or whether cooling of the metalshell is insufficient. Steps are then taken during furnace operation,based on the results of such calculations, to fill such gaps withrefractory, to re-establish sufficient cooling of the shell or to form aprotective solidified layer of sufficient thickness on the inner surfaceof the refractory. The method of this invention also includes performinga moving boundary calculation directly from measured temperatures at allthermocouple locations in a vertical plane simultaneously to discernirregular erosion profiles, e.g. "elephant-shaped" and "bowl-shaped"erosion of the refractory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of one-half of a blast furnace hearth showingthe arrangement of thermocouples in the refractory and metal shell ofthe hearth area of the furnace.

FIG. 2 is a schematic side elevation view of the tap hole area of ablast furnace illustrating the placement of thermocouples in that area.

FIG. 3 is a flow diagram of steps taken in accordance with the method ofthis invention.

FIG. 4 is graphic representation of blast furnace hearth refractorylining wear and solidified metal skull formation determined according tothe method of this invention.

FIG. 5 is a schematic representation of two refractory-metal shellinterface temperatures, TIR and TIS, determined by two differentanalyses, with TIR substantially greater than TIS, indicating thepresence of a gap between the refractory and metal shell.

FIG. 6 is a schematic representation of the two refractory-metal shellinterface temperatures, TIR and TIS, with TIS substantially greater thanTIR, indicating insufficient cooling of the metal shell.

MODES FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, thermoprobes for measuring temperature, preferablythermocouples, are embedded in the refractory and the metal shell of ablast furnace in the hearth area. Thermocouples 10 and 12 are placed inthe refractory sidewall of the hearth at two known positions across thethickness in a radial direction of the furnace. At least onethermocouple 14 is embedded at a known position in the metal shell inline with thermocouples 10 and 12 in the refractory. This first group ofthermocouples is preferably placed at the elevation of a tap hole of thefurnace and in the vicinity of the tap hole. A second group ofthermocouples 16, 18 and 20 is placed at substantially verticallyaligned positions with the first group at an elevation above the topsurface 21 of the hearth pad. A third group of thermocouples 22, 24 and26 is placed at substantially vertically aligned positions with respectto the first two groups at an elevation in the corner of the hearthsidewall where the sidewall meets the hearth pad. Thermocouple groupsare placed in the floor of the hearth i.e. in the heart pad, at twoknown elevations with their hot junctions vertically aligned. One pairof thermocouples 28 and 30 is placed in the centerline of the furnace. Asecond pair of thermocouples 32 and 34 is placed one-third of the way tothe inside of the metal shell at the hearth sidewall. A third pair, 36and 38 is placed two-thirds of the distance to that location. It ispreferred that thermocouples in this arrangement be placed spacedlocations around the periphery of the furnace, with thermocouplecoverage concentrated in the areas of the potentially highest wear suchas around the tap hole.

Referring to FIG. 2, the placement of thermocouples around the tap holearea of the furnace is illustrated more specifically. Two groups ofthermocouples 40 and 42 are located on either side of the tap hole atabout the tap hole elevation. These thermocouples may be in-line in ahorizontal direction normal to the plane of the drawing or spacedperhaps spaced about four inches (10 cm) from each other in the verticaldirection as shown in FIG. 2. At an intermediate elevation above the topsurface of the hearth pad, five groups of thermocouples 44, 46, 48, 50and 52 are located in the tap hole area. Another five groups ofthermocouples 54, 56, 58, 60 and 62 are located at about the elevationof the corner of the sidewall and the top surface of the hearth pad. Thethermocouples in the latter two groups may be spaced about two feet(0.6096 meters) apart in a horizontal direction in the plane of FIG. 2.

As shown in FIG. 3, the readings taken from the plurality ofthermocouples embedded in the refractory and shell are used as input ina computer program to carry out a sequence of heat transfercalculations. The current average and campaign maximum temperatures fromthese thermocouples are fed into a heat transfer model that translatesthese inputs into a heat flux. From this information, a one-dimensionalheat transfer model calculates the location of the hot metalsolidification isotherm, for example 2100° F. for blast furnace iron.The solidification isotherm which results from this calculation is usedas the initial boundary in a two-dimensional heat transfer model. Thetwo-dimensional heat transfer program iterates until a final boundary ofthe solidification isotherm is determined by minimizing the differencebetween the measured and predicted temperatures at each measuring point.The two-dimensional heat transfer calculations are made on the basis ofthe following two equations: ##EQU1##

where T is the temperature at the location where the radius co-ordinatefrom the centerline of the furnace is r, and the height co-ordinate isx. By using the average and the maximum temperatures, the sequence ofheat transfer calculation provides two solidification isotherminterfaces, Ih and Ic, respectively, as shown in FIG. 4. The interfaceIh is closer to the hot side as compared with Ic. The isotherm interfaceIc, which is closer to the cold side represents the wear profile of therefractory lining, while Ic represents the skull formation between hotmetal and the lining. The distance between Ih and Ic represents thethickness of the solidified skull. The presence of the skull preventsthe lining from the direct attack by hot metal and enables extending thelife of the blast furnace hearth. The beginning of an irregular erosionprofile in the corner are is illustrated in FIG. 4, where an"elephant-shaped" erosion is beginning to form as indicated by a greatererosion 62 in the corner than the erosion 64 in the adjacent sidewall.The method of this invention is able to determine "elephant-shaped"irregular erosion in an on-line calculation using a moving boundarycalculation which considers interaction between measured temperatures atall locations in a vertical plane simultaneously.

The campaign maximum thermocouple readings represent the ultimateerosion profile of the refractory brick. Generally, the campaign maximumreadings correspond to the highest heat flux and minimum calculatedrefractory thickness. These readings are then applicable to determinethe critical isotherm corresponding to the actual erosion profile of thebrick, Ic.

The average thermocouple readings represent the current condition withinthe hearth. The last hour's average temperatures recorded at eachlocation is used to determine the current position of the criticalisotherm and to calculate TIR and TIS, for purposes describedhereinbelow, at all locations in the furnace hearth. The position of thecurrent critical isotherm, Ih, relative to the isotherm corresponding tothe maximum calculated refractory erosion profile, Ic, relates thepresence and relative thickness of the protective skull on the insidesurface of the refractory at each location. In general, when the currentaverage thermocouple readings approach the campaign maximumtemperatures, Ic˜Ih, this indicates that there is presently noprotective skull layer on the inside surface of the brick.

After calculating the critical isotherms, at each location, calculatedvalues of TIR and TIS are used to determine the presence of a gapbetween the shell and the hearth brickwork and the presence of abuild-up on the furnace shell. ##EQU2##

Where

TIR =Calculated shell interface temperature as calculated from hot side

TIS =Calculated shell interface temperature as calculated from watercooled side

T₁ =Measured temperature from location 1, see FIGS. 5 and 6

T₂ =Measured temperature from location 2, see FIGS 5 and 6

T_(s) =Measured shell temperature

L_(i) =Relative position of shell interface as referenced from thecoordinate system

L₁ =Relative position of thermocouple 1 as referenced from coordinatesystem

L₂ =Relative position of thermocouple 2 as referenced from thecoordinate system

L_(s) =Relative position of shell thermocouple as referenced from thecoordinate system

k₁ (T)=Thermal conductivity as a function of temperature betweenlocations 1 and 2

k_(i) (T)=Thermal conductivity between shell interface and location 2

k_(s) (T)=Thermal conductivity of the shell

Given these calculated values, the program then uses logical comparisonstatements to indicate potential warning conditions and directsappropriate action to alleviate any problems indicated.

As shown in FIG. 5, TIR and TIS are calculated and then compared todetermine if a gap has formed. TIR is calculated according to theformula above from temperatures T₁ and T₂ at thermocouple locations 1and 2 in FIG. 5 (which correspond, for example, to thermocouples 10 and12 in FIG. 1). TIS is calculated using the formula above fromtemperatures Ts, T₁ and T₂ temperatures (where Ts corresponds tothermocouple 14 in FIG. 1). The following relation is used for thatcomparison:

If (TIR-TIS)>Preset limit (50° F. in one case), then a gap has formedbetween the refractory and the metal shell. The preset limit typicallymay be selected from within a range of from 20 to 120° F. Correctiveaction may be taken to fill the gap e.g. with a high conductivity groutmaterial to re-establish contact with the cooled shell.

As shown in FIG. 6, TIR and TIS are calculated and then compared todetermine if a build-up on the shell has occurred. The followingrelation is used for that comparison:

If (TIS-TIR)>Preset limit (50° F. in one case), then the water coolingon the shell is insufficient. Again the preset limit typically may beselected from within a range of from 20 to 120° F. Action may be takento check for a problem in the water system or to determine if apotential build-up has formed on the outside surface of the shell whichis interfering with proper heat transfer. After determining the cause ofthe problem, action may be taken to remove the build-up or tore-establish proper water flow in this area to improve heat removalefficiency. Conventional measures may be taken to correct theseproblems, for example, the shell surface may be sand blasted to removeany build-up or other measures may be taken to correct insufficientwater flow or high temperature water.

Where there is no gap formed between the refractory and the shell andwhere cooling of the shell is sufficient and yet there is no solidifiedmetal skull formed on the refractory lining of the furnace, or thethickness of the skull is insufficient to serve as protection for therefractory, measures may be taken to form a solidified metal skull ofsufficient thickness to serve as protection for the refractory or toform additional refractory on the surface of the lining. Such measuresmay take the form of injecting or charging titanium-bearing materialsinto the furnace to protect the inner surface of the hearth, or reducingproduction and adjusting tuyere velocity to form a solidified metalskull of sufficient thickness.

Industrial Applicability

The invention is applicable to blast furnaces for producing iron for thesteel industry as well as in blast furnaces for producing non-ferrousmetals.

We claim:
 1. A method of determining the thickness of a protective layerof solidified metal skull formed on the refractory hearth of a blastfurnace wherein the refractory hearth has temperature probes embedded atspaced locations in radial directions from the center of the furnace andat various elevations across the thickness of the floor and wallsthereof, comprising the steps of:a. periodically measuring temperaturesat said spaced locations in said radial directions and across thethickness of the furnace hearth floor and walls by the temperatureprobes embedded therein; b. determining the maximum temperature recordedby each temperature probe since the beginning of a campaign of thefurnace and the average temperature recorded by each temperature probeduring a current time period; c. analyzing the relation of said campaignmaximum and current average temperatures of said temperature probesembedded in the hearth walls correlated with the radial distance betweenthe temperature probes and the center of the furnace and the relation ofsaid campaign maximum and current average temperatures of saidtemperature probes embedded in the hearth floor correlated with theelevation distance between the location of the temperature probes in thehearth floor so as to predict the location of the wear line of therefractory hearth from the location of a solidification isotherm closestto a metal shell, and to predict the location of the inner surface of asolidified metal skull lining said refractory hearth from an isothermclosest to the hot side of the furnace remote from the metal shell, andd. determining the thickness of the protective layer of solidified metalskull from the distance between the predicted wear line of therefractory hearth and the predicted inner surface of the metal skull. 2.A method according to claim 1 wherein said analysis is carried out byestimating the location of said solidification isotherms using a onedimensional heat transfer approximation, using this approximation as theinitial boundary to begin a moving boundary calculation from a twodimensional heat transfer model, and continuing to iterate the twodimensional heat transfer model until a final boundary of eachsolidification isotherm is determined by minimizing the differencebetween the measured temperature at each temperature probe location anda predicted temperature at said location based on iterations of the twodimensional heat transfer model.